Money-saving suggestion: Buy a 'day Trip' ticket from London to Calais, even for a one-way trip... It's very poorly publicised (it's not even mentioned on their website!) but South Eastern Trains offer a day return day tripper ticket from London to Calais for £33.90 adult, £16.95 child, or £22.35 for Senior or Young person railcard holders (2011 prices). It's valid on any train to Dover even in the morning peak and on any P&O ferry to Calais, no reservation necessary. You can buy it at Charing Cross or Victoria stations on the day of travel. It's intended for day trippers, but even for one-way trips this £33.90 train+ferry ticket is a lot cheaper than buying a £34 one-way train ticket to Dover plus a £30 foot passenger ferry fare to Calais. There is no problem buying one of these day returns and not using the return leg. Just throw the return half away! Indeed, if you can buy a London-Calais day return ticket in advance in the UK for the date you want to come back, you could probably use the return leg of a second day trip ticket as your inbound one-way from Calais back to London. The ticket can also be bought starting in many other stations in Kent & South London such as Croydon, Guildford, Woking, Orpington, Canterbury or Ashford. It doesn't cover the bus between Dover Priory and Eastern Docks. You can confirm prices & details by calling South Eastern's customer services on 0845 000 2222.
